The present invention relates to communications synchronization and particularly to time-alignment of transmissions of a mobile station in relation to the time reference of abase station in a mobile radiotelephone system.
Presently, much attention and developmental effort is being focused on digital mobile radiotelephone systems. Typically, the mobile stations and the respective base stations of such a system communicate across TDMA channels whereby multiple mobile stations share the same transmission and reception frequencies but are assigned separate time slots within those frequencies. Each mobile station must accurately confine its transmissions to the bounds of the time slot assigned. Otherwise, transmissions from different mobile stations "collide", resulting in mutual interference in reception at the base station.
Accurate time-alignment becomes especially important as the cell size of a cellular system increases, since the maximum time dispersion at the base station of transmissions from the mobile stations likewise increases. Time dispersion refers to the different propagation delays of different transmitting mobile stations located at different distances from the base station. As the mobile stations move, the effect at the base station of different varying propagation delays is compensated for at the mobile stations by the base station commanding the mobile stations to adjust their transmission timing. Time slots therefore occur at the base stations in regular order without any apparent effect from the different propagation delays.
Techniques have been developed to keep the transmissions of the respective mobile stations in proper time-alignment. Typically, while communications are being established between a mobile station and a base station, the mobile station times its transmissions in relation to transmissions received from the base station according to a "standard off-set reference". In one proposed system, for example, the mobile station times its transmission such that transmission power is dying out at the conclusion of the transmission 42 symbol periods before the next expected communications burst from the base station. Until the base station has assigned the mobile station a specific transmission time, the mobile station transmits a shortened burst of shorter duration than normal to minimize the likelihood of colliding with other transmissions.
Once a time slot has been assigned to the mobile station, time-alignment of transmissions by the mobile station is continually checked by the base station by noting the time of arrival of a synchronization pattern transmitted by the mobile station during each burst. Time drift of transmissions by the mobile station with respect to the clock of the base station occurs due to time dispersion. When that drift exceeds a predetermined threshold, the base station sends a time-alignment command to the mobile station instructing it to advance or retard its transmissions by a specified amount of time.
The synchronization pattern transmitted by the mobile station at each burst may also occur randomly in normal speech data. Therefore, to avoid spurious detection of apparent synchronization patterns ("false syncs"), the base station ignores synchronization patterns not occurring within a specified time interval, or "window", of where they are expected to occur.
When the base station issues a time-adjustment command to the mobile station, the problem occurs that the base station cannot tell exactly when the mobile station will have responded to the command and have begun transmitting according to the adjusted timing. For a short time, therefore, the sync pattern may occur either at the old timing or the new instructed timing. In order to provide for both possibilities, in the prior art, the time-alignment window wherein synchronization patterns are recognized has been stretched, or widened, to cover both the old timing position and the new timing position. With the widening of the time-alignment window, the probability of detecting false syncs increases, and speech quality decreases proportionally.